Hi Ian,
One possible cause is you have the ReCaptcha plugin enabled but have no public & private key entered.
http://tamlynsoftware.com/contact-us/support-tickets/bf-quiz/4-error-500-recaptcha-plugin-needs-a-public-key-to-be-set-in-its-parameters-2.html

If that's not it, set error reporting to maximum in your global config, and see if you get an exact error message that will help determine the cause.

regards

Tim

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